Application No. P 33 33 755.1 The subject of the not yet published German Pat. (U.S. patent application Ser. No. 640,607--filed 8/14/84) is a special magnetic device of a system for nuclear spin tomography which contains several magnet coils which surround an interior suitable for receiving a body to be examined and which produce therein a magnetic field which is at least largely homogeneous. The magnet coils are surrounded by an approximately hollow-cylindrical shielding device of ferromagnetic material. This shielding device has at its respective end faces a disc-shaped plate with a central opening having a predetermined radius relative to the cylinder axis, wherein four beam-like shielding elements of identical design extend, regularly distributed in the circumferential direction, over a common cylinder surface, the cross sectional areas of which are triangular or trapezoidal, and each of which comprises a base area facing the magnet coils and two orthogonally aligned side surfaces adjacent thereto.
Nuclear spin tomography systems (Nuclear Magnetic Resonance (NMR) Systems) for medical diagnostics are generally known (see, for instance, European Pat. No. 21 535 A1 or DE-OS No. 29 21 252). Such installations comprise a magnetic apparatus with an arrangement of normal or, in particular, superconducting field coils, by which a base field as strong as possible can be produced which has adequately high homogeneity in a measuring range. On this base field are superimposed pulsed gradient fields. In addition, a high-frequency field oriented perpendicularly to the base field must be provided.
The dimensions of the corresponding coils must be adapted to the dimensions of the body to be examined in such a manner that the latter can be inserted without problem into the measuring area of the interior surrounded by the coils.
The strong magnetic base field of such a coil arrangement should substantially be limited to the measuring area but should not spread, as far as possible, in the form of a stray field, into the surroundings of the coil arrangement, where it can lead, for instance, to disturbances in certain electromagnetic equipment or can also exert an undesirable force effect on ferromagnetic, especially moving, parts. Magnetic apparatus for nuclear spin tomography therefore advantageously comprises, besides the coil arrangement, also a separate shielding device in order to keep the stray field outside the coil arrangement limited (see DE-OS No. 32 45 945).
With the shielding device proposed by the mentioned patent application, a far-reaching return of the magnetic flux around the coil arrangement of the magnet device can be ensured without the need for closed cylinder surfaces, since such closed surfaces are undesirable expecially if superconducting coils are used. With such coils, lateral openings are required, for instance, for the so-called terminal tower of the necessary cryostat as well as for pump nozzles. In addition, lateral access for an adjustment possibility of the coils for correcting gradient fields should be provided. Accordingly, the cylindrical surface is resolved in the proposed shielding device into four beam-like shielding elements of identical design, which extend between two disc-like plates at the end faces and are arranged, distributed regularly in the circumferential direction together on an imaginary cylinder surface. Even so, the stray field, for instance, of a 1-Tesla magnet can be reduced by such a shielding device by a shielding factor of about 4 to 5.
While with the proposed shielding device, good lateral access to the cryostat and the magnet coils is assured due to the gaps formed between the beam-shaped shielding elements, the concentration of the shielding material in these four shielding elements leads to the situation that the magnetic field in the useful volume suffers a four-numbered field distortion, since under the shielding elements, the field at the edge of the useful field is somewhat greater than in the regions of the gaps between these elements. The four-numbered axis of the shielding is therefore still noticeable in the useful volume as a fourth-order azimuthal error (in a plane perpendicular to the cylinder or magnet axis).